Identifying Flood Hazards

Identifying Flood Hazards

The first large-scale flood hazard identification engineering analyses conducted within the Navajo Nation will help reduce potential loss of life and infrastructure and property damage for more than 300,000 people.

By David Turk, GISP, CFM, M.SAME

3D view of flood hazards in Kayenta, Ariz. Navajo Nation has endured many social and economic challenges over the years, including the loss of property and residual health and safety risks associated with flood damage. IMAGES COURTESY URS CORP.

Navajo Nation, over the years, has endured many social and economic challenges. These include the loss of property and residual health and safety risks associated with flood damage. The flooding problem is complicated by the fact that Navajo Nation is a huge expanse, with varying topography covering some 27,000-mi² and spanning three states, Arizona, New Mexico and Utah.

In an effort to address the flood risk issue, URS Corp. designed and implemented a project to provide the Navajo Housing Authority (NHA), with floodplain information and a hazard mitigation plan that it previously lacked. NHA is the Navajo Nation’s Tribal Designated Housing Entity pursuant to the requirements of the Native American Assistance and Self-Determination Act of 1996, as amended. NHA manages more than 9,000 homes across the Navajo Nation. It is the largest Indian housing authority in the country and is approximately the tenth largest public housing authority in the country next to the City of Atlanta Housing Authority.

IDENTIFYING VULNERABLE AREAS

The project enabled NHA to conduct the first large-scale flood hazard identification engineering analyses within the Navajo Nation. The latest automated modeling techniques were used to perform extensive engineering analyses and identify the areas most vulnerable to flooding. Given the Navajo Nation’s topographic variability, the landscape ranged from low-lying alluvial valleys to high canyon arroyos. To account for the varying landscapes and soils, a variety of innovative techniques were used for engineering and GIS analyses.

The project began with the identification of flood-prone areas that are home to more than 300,000 people in the Navajo Nation. The first priority was mapping the 1 percent annual chance floodplains for a large portion of the population, identifying risks from flooding and the potential for loss of life and property. New technologies were developed and used to provide accurate and cost-efficient floodplain mapping products with tight timeframes to meet NHA standards. Using software approved by the U.S. Army Corps of Engineers (USACE) and the Federal Emergency Management Agency (FEMA), automated floodplain studies were conducted for the 3,000 stream miles, and more than 480 floodplain maps were produced.

The flood hazard data aids NHA in developing floodplain management policies and making informed decisions about future planning, development and investment in public infrastructure, land use and home construction. The development of flood hazard information enhances mitigation efforts and response to disaster. With flood hazards identified, the Navajo Nation chapters are better prepared to manage flood risks, land use, water resources, and their disaster recovery assets and resources.

In September 2013, the Navajo Nation experienced heavy rains that led to flooding. Flood hazard information derived from this project was used to assess damage and determine clean-up efforts.

INNOVATIVE TECHNIQUES

Innovative modeling techniques were devised to meet the complexity of client needs throughout the project.

Watershed-Based Engineering Analyses. While floodplain studies are typically analyzed based on community boundaries, a watershed approach was used to identify flood hazards across Navajo Nation. This technique was evidence-based rather than jurisdiction-based; the evidence was derived from research provided by FEMA and other floodplain management authorities. Instead of confining the estimate of flood risk to individual community boundaries, the watershed technique allows for a more holistic and accurate method of identifying and understanding a community’s flood risk.

3D view of Flood Hazards Depth Grids in Shiprock, N.M., Navajo Nation. Understanding the boundaries of the floodplain and protecting it from development are key first steps in establishing land policies that will ultimately lead to healthier ecosystems.

Hydrologic Unit Codes (HUCs) are numeric sequences used to identify watersheds. URS and NHA determined that studying flood hazards at the U.S. Geologic Survey (USGS) HUC-12 breakdown was the best way to proceed. The project area was selected using sites identified by NHA. USGS HUC-12 sub-watersheds that contained the selected sites were chosen as the project limits. HUC-12 sub-watersheds have average areas of about 40-mi² and are comprised of a segment of a river and areas that drain into that segment.

Proposed Floodplain Management Plan. It was clear that simply identifying flood hazards would not be sufficient to meet NHA’s goal of building sustainable communities in the context of flood hazard mitigation. Therefore, floodplains were identified and the information used to create a Flood Hazard Mitigation Plan (HMP).

As part of the HMP, floodplain management policies for NHA were proposed—including policies to guide future development away from identified flood hazards and for the remediation of homes located inside the floodplains. A two-pronged approach for safe and quality home construction within the Navajo Nation was proposed. The first is a Freeboard Policy, requiring 2-ft vertical offset above the flood elevation as a factor of safety. The second aspect is a Horizontal Setback Policy, requiring a horizontal buffer along the floodplain to account for potential erosion.

Flood Depth Grid Map Generation. The team produced 1 percent annual chance depth grid maps for all 3,000 stream miles. Depth grid maps display flood risk by showing the variable flood depth across the floodplain as opposed to a one-percent annual chance flood elevation. A flood depth grid clearly relays the variability and severity of flooding for individuals and communities unfamiliar with the technical language associated with flood elevations.

This results in clearer communication of flood hazards to NHA and its communities. Depth grid data was created automatically by subtracting the ground elevation from the water surface elevation. This process primarily used the RAS Mapper tool in USACE’s HEC-RAS software, saving both time and money.

3D Outreach Videos. An effective community outreach strategy was necessary to implement the engineering, mapping and mitigation policies. Several 3D fly-through videos in both English and Navajo were created that took into consideration Navajo tradition and culture. The educational videos displayed identified flood hazards over NHA’s imagery and topographical data.

BRIDGING THE GAP

The Navajo Nation floodplain mapping and hazard mitigation project required the bridging of a cultural gap in relations between URS and NHA. Few tribal entities have engaged in flood hazard mitigation due to various funding and governance complexities. This project’s design and implementation processes called for the introduction of novel contracting mechanisms that enabled collaboration with a tribal entity; the navigation of complex political environments; and the negotiation of legal terms and corporate risk management not ordinarily associated with non-tribal entities.

Understanding the boundaries of the floodplain and protecting it from development are essential for establishing land use policies that promote sustainable development practices that will lead to healthier ecosystems. By conducting flood mapping in areas that had never undergone analysis or mapping before, the project team provided the Navajo Nation with critical information to be used in reducing potential loss of life, and infrastructure and property damage. With the creation of floodplain maps and flood-risk reduction products, NHA can make better-informed decisions in planning for community sustainability and economic prosperity.

David Turk, GISP, CFM, M.SAME, is Project Manager, URS Corp.; 505-855-7543, or This email address is being protected from spambots. You need JavaScript enabled to view it..